Radio station selector



B. A. SCHWARZ RADIO STATION SELECTOR 3 Sheets-Sheet l NNN Filed Jan. 19, 1948 Sept m 1950 B. A. SCHWARZ 2,521,752

RADIO STATION SELECTOR Filed Jan. 19, 1948 s sheets-sheet 2 Sept. 129 1950 B. A. SCHWARZ RADIO STATION SELECTOR Filed Jan. i9, 1948 3 Sheets-Sheent 5 levi I A y I wlw;

IMM

lmentor Patented Sept. 12,` 1950 UNITED STATES PATENT oFFlcE Bertram A. Schwarz, Kokomo, Incl., assigner"toY General Motors Corporation, YDetroit, Michl., 'a

corporation of kDelaware Application January 19. 1948, Serial No. 3,050

l5 Claims.

The present invention relates to radio receivers capable of receiving a signal modulated at-b'ofth (ci. 25o- 20) audio (voice or music) frequencies and a submanual tuning, does not have any 'selectivity ex-V cept that introduced by the operator himself. A second method or" tuning using a vso-called push button, either sequential or selective, has certain preselected frequencies set up on the mechanism for specific stations. A third method of tuning, that of signal seeking selective 'tuning includes a means for automatically tuning in any station having a predetermined signal strength. None of the various methods of tuning solve the problem encountered by the radio operator desiring to tune` to any station of a Apredetermined system of stations.

In the practice of the present invention it is provided that each station of a chain carry a vsub'- audio code signal modulated on the carrier. The modulation of this code signal may be either amplitude modulated or yfrequency modulated. For the purposes of illustration the gures in the present patent application and the specificati-on thereof will be particularly concerned With amplitude modulated signals having a sub-audio code signal amplitude modulatedv thereon.

It is therefore an object of the present invention to produce a radio receiver havingr an auto matic tuning means selective to receive only signals having a specic code frequency modulated thereon.

It is a further object of the present invention to produce an automatic tuning means for a radio receiver, capable oi sequentially tuning the receiver to all signals, within the frequency spec.- trum, of such amplitude as will give satisfactory reception.

It is a further object of the present invention to produce an automatic means capable of tuning a radio receiver selectively to any station having a preselected code frequency modulated on its carrier and having a signal strength of suiiicient intensity to give satisfactory reception.

. .2 It is a further object of the pre'sentvinvention to produce a vsqueicn circuitcapable of making the'audio vamplie'r section of a radio receivery insensitive to all signals vexcept those carried by a vcarrier frequency having modulated thereon Va code frequencyof preselected sub-audio frequency and vone yin which the signal is of sucient inftensity to give satisfactory reception.

It is a further object or" the present invention toY produce a niter system for selecting the code frequencylit isa further object of the present invention to produce a `'receiver which will differentiate between vthe different chain radio systems and "i tuneV in yonly ytn'at'station carrying la preselected chain system program and having suitable sign-al intensity for satisfactory reception.

It is a further 'object of the present invention to produce a receiver with means for squelching 'all signals except'those having sufli'cient intensityto givesatsfactory reception. v 4

' Other objects' of this invention will become apparentupon readingT the specification and inspection of the-drawings and wil1 be particularly pointed out in the claims.

Referring to the 'gures:

Figure 1 is a manually 'tuned radio receiver with a squelching circuit.

Figure 2 is a motor tuned radio receiver. Figure 3 is a mechanical vibrator it'er system. vReferring more particularly to the gures,=Fig ure 1 lshows a manually tuned radio receiver having means for squelchlng all signals except those carrying a preselected sub-audio code modulation. In this ligure; 2 is an antenna of 'convene tional design and 4 'is a radio signal receiver in cluding the R. F. amplifier, I. F. amplifier 'and detector circuits. The circuits of the R.. F. amplier, oscillator, and most of the I'. F. amplifier, are not shown since these circuits are of conventional design. 6 is an amplifier capable of filtering out the audio frequencies and` amplifying 8, mand I2 are' the sub-audiocode frequencies. selective band `pass filters, each capable oi passing only the' sub-audio code frequency selected.

M'con'tans an amplifierdetector'-, squelch circuit and audio frequency amplie'r, lo is a manual tuning'd'evlce which .may be a knob of other conventi-cual means cfm-anual-l'y adjusting the resonant circuits of thev receiver.

Referring to the amplifier 'and detector ini-- design and therefore not illustrated. The `plato li.. v.- 1J. 3

circuit of this tube includes the fixed frequency tuned primary circuit 20. The coil 2| in this circuit is inductively coupled to the coil 23 of the resonant secondary circuit 22. The circuits 20 and 22 form a double tuned circuit for the last intermediate frequency stage. The circuit 20 is coupled to the rectier tube 26 by means of condenser 24. This circuit which is of conventionaldesign is used to produce a D. C. automatic volume control voltage across the resistor 28. The secondary tuned circuit 22 is coupled to the audio frequency detector 30. This circuit in common with the AVC circuit contains a biasing resistor 32 and by-pass condenser 34. By means of this circuit and resistances 36 and 31 audio and sub-audio frequencies are produced across these latter two resistances. Audio and sub-audio frequencies of high intensity are transmitted by conductor 40 to the amplifier 6.

This amplier contains a filter including resisto1-42 and condenser 44 for by-passing and removing all audio frequencies. The sub-audio frequencies appearing at the point 45 are transmitted to the grid 50 of the tube 52 by a coupling circuit including condenser 46 and resistor 48. The value of the power factor of the circuit including resistor 42 and condenser 44 is such that all frequencies below the audio range including the sub-audio code frequencies are impressed upon the grid of the tube 52. 54 is a plate load resistor for tube 52. The negative grid bias for thistube is obtained through a circuit including biasing resistor 58. The sub-audio code frequencies produced at the plate of this tube are coupled to the band pass filters by condenser 50 and resistor 62. The sub-audio code signal present at the points G4 is simultaneously impressed upon the filter systems 8, and I2.

Each of these filter systems is a sharp cut-off band pass lter responsive to a preselected subaudio signal. For example, the filter system 8 will utilize such values of condensers 65, 66 and 68 and inductances 10, 7| and 'I2 to pass a preselected sub-audio signal of, for example, 5 cycles per second. The filter system I0 will have such value of condensers 14, 16 and 'I8 and inductances 80, 8|l and 82 so as to pass a frequency of, for example, 10 cycles per second. The filter system |2 will have such value of condensers 84, 86 and 88 and inductances 90, 9| and 92 so as to pass a frequency of, for example, 20 cycles per second.

The selection and arrangement of the condensersy and inductances in each of these filter systems will depend on the sub-audio frequencies used by the chain system program which he desired by connecting the corresponding lter system into` the circuit. Each of the systems has a push button with contacts thereon capable of connecting that lter system into the circuit. The -push buttons 94, 96, and 98 connect the filters 8, |0, and

I 2 respectively into the circuit.

The selected sub-audio frequency proceeds through the conductor |00 to the grid |072 of the tube |04. |03 is the ground return resistor for the grid |02. When the radio receiver is tuned to any frequency other than that frequency having the selected coded sub-audio fre quency thereon, the line or conductor |00 carries no signal, and the tube |04 is in a passive state. The tube |04 has a cathode |06 which is self-biasedby a resistorV |05and I,condenser |01, atriode plate |08 and two interconnected diode plates ||0 and |l2. When a sub-audio signal is impressed on the grid |02 the triode amplifies this signal across plate load resistor ||4 and the signal is fed through coupling condenser I6 and is impressed across the diode by means of resistor H8. Theoutput of this diode rectier system is a combined negative D. C. and negative rectified pulses of the selected sub-audio frequency. The A. C. components of this signal are removed by the low pass filter circuit including resistance |20 and condenser |22. The power factor of this filter circuit is so selected that it will effectively attenuate all sub-audio frequencies which may be used as code or control frequencies. The remaining negative D. C. component is impressed upon the grid |24 of the tube |26. This tube in addition has cathodes |28 and |30, plates |32 and |34 and a second grid |36. When the grid |24 has suicient negative potential impressed thereon to bias that half of the double triode to below cutoff no current will flow between the plate |32 and the cathode |28.

Each half of this double triode has an adjustable grid bias resistor in the cathode circuit. These are resistors |38 and |40. The adjustment of these resistors determine the amount of negative potential across 36, 31 and ||8 respectively necessary to bias their respective triode to below cutoff. Since this tube handles only D. C. potentials it is not necessary to have an A. C. bypassing condenser across the biasing resistors. A second squelch removing signal conductor |42 is connected across the resistors 36 and 3l to the same point as conductor 40. The signal at this point is a combined audio and sub-audio signal superimposed on a D. C. potential. The subaudio and audio frequencies are removed by the low pass filter circuit including resistor |44 and condenser |46. The power factor of this circuit, the same as mentioned in connection with the power factor of the circuit |20, |22 must be such as to attenuate all sub-audio frequencies which may be used as code or control frequencies. The unattenuated negative D. C. potential is then impressed upon the grid |36 through the conductor |48. When this negative D. C. potential is sufficient to bias the triode to below cutoff no current will flow between the plate |34 and the cathode |38. Here, as mentioned before, the adjustment of the self-biased resistor |40 will determine the potential at which the tube is biased below cutoff and therefore non-conducting. The biasing potential for the bias resistors |38 and |40 is obtained through potential dropping resister |50 and |52 from the plate supply potential or B plus source |60 of the radio set. It may thus be seen that when both grids |24 and |36 are biased below cutoff no current will ow through the conductor |54. However, if either of these grids are of such potential as to allow tube conductio-n, current will flow in the conductor |54. Therefore, to prevent flow of current in this conductor |54, the signal received must contain the sub-audio frequency selected by one ot the push buttons 94, 96, or 98.

It may at times be desirable to receive local aanwas ence of. an .audio frequencyandlu C, in the :con-y ductor |42. An alternate loc-ation fortherswitch |56.is switch |58. On opening switchf-|53iboth squelch removing signal `circuits `willi-befrendered4 inoperativeaswill be appreciated as this .speci-A cation progresses. Assuming switches |56. and

|58 toA berclosed and nc audio or.sub-audioiiraV4 quency present, a current `from the source |60 flowslthroughlconductors |62 .and |54 and re sistorlli. Current at the :point |60 :may :divide part 'of thiscurrent flowing throughthe resistor |;to give. a plus `bias 'on .the f. cathode |14. This positive biasis added 'to the audio signals@ that itlifts the entire operating potential ofthe tube |12. but vdoes not affect :the relative .potentials between the :cathode |14,.and .grid-.1.1.6 Lof.A this tube...v Thecurrent-.at thepoint |60 .may proceed through squelching resistor |80 as currentrlows.

through the conductor |54 when eithenhalf of the double .triode |26 is conducting.. vWhen currentA flows .through ,this resistor, the. point .1| 82.is biased to-:a .potential negative relative to the point |60..

This .ineffeot places a negative .potential on the grid .|16and thus biases this .grid tobelow its cutoipoint so that no signal will be .amplified bythe tube. The audio frequency ,which tubel .12.

ampliiiesis obtained .iromatap .on the .volume control Aresistor 31 and is .coupled throughcon" denser |64 to `conductor |86. IIhissignal pressed across resistors |88, |80 and |10. If the grids-|24 and |36 are biased `beyond-cutoiso that ,no current flows inconductor |54 orifthis.

saine result is obtained by the opening of switches by resistor |90 andcondenser |92 so` thatit is atV thefproper operating .potential yofy lthis tube. Under/this set ofconditions tube |12acts as 'an amplierand an audio frequency is impressed across load .resistorv led-and through couplingr condensersi and |98 across grid.resistor 206y tov the grid 202 of the beam power tube,2|l4.,y Thei .tube 204 has a cathode 206 whichisself-biased byV resistor 208and condenser 2| 0. The lgrid 2|2 of Vthis tubeand theplate 2 |4 bothZ have the full plate potential obtained from source l`-|60 .iinpressed thereon. The couplingvv condenser v|98 in one particular modification of the` present inventionis made adjustable for tone control purposes. L4The varying of ycondenser |98 changes thelltime constant of the circuit inoludingrcondenser |96 and resistor 200 so as to attenuate the low audio frequencies an amount dependent upon the adjustment of this condenser |98. The audio frequency signal impressed upon the control grid 202 is amplified by the tube204 and impressed acrossthe primary 216 of the audio frequency output transformer 2|8. The output from the u secondary 220 of this transformer is `impressed across the voice coil 222 of theloud speaker. This speaker may be of either the permanent magnet type, or electromagnet type as it'iorms no part of' the present invention. l If it' is of the electromagnet type, magnetic coil 2.24 is required for energization.

Operation of circuit in Figure 1 tem program, he will press the selectivebutton of' 94 which..couples the band pass .-lterfzforwthe National Broadcasting System sub-audio fre quency into the circuit. The operator thenfturns the manual tuner' I6 'scanning thev broadcast band. As various radio frequency rsignals are received theyare rectied and nltered `andthe remaining D. C, component is fed throughwconductor |48.to energize the grid |36fcausing the current between plate |34 and kcathode |30 to be interrupted. This,`however, does .notfintery rupt the current through the conductor i154 =so as to removethe squelch signalfrom the grid |16v since current is allowed to flow between theplate |32 and the cathode |28 of the tubeA |26.'` The operator continues the operation of the tuner knoby I6 until a signal is reached containing the sub-audio vfrequency which the bandpass lterl will pass. When this happens, a signal'is-.con

ducted through conductor |00'to the grid |02 of= the tube |04 where it is amplified and fed to .thel plates I0 and ||2 of the diodev portion of thisl tube. The detected loutput is then fed through..

a lter and the remaining negative D. C. potential impressed on the grid |24 .to interrupt the, current flow between the plate |32 and the "cath'l f' ode |28. At the same timeia second current is conducted throughfconductor |42 and filtered 'so' that its remaining negative D. `C. potential `vis' A impressed upon the grid |36 interrupting theflovv of current between the plate |34. and the cathode' |30, thus current flow in the conductor |54iis interrupted so that the D. C. bias betweenthe points |82 and |68 is zero. In this condition the bias potential on the grid |16 'is such as to place'Y this grid at itsoperating potential rand vpermits amplification-of audio frequency potentials.A 'The audio lfrequency signal is conducted through conductor |86 amplified by tubes |12 -and 264 and ultimatelyimpressed across the voice coil 222 of the loudspeaker thus energizing thespealser." It may thus .be seen that when the switches'vl58. and |56 are closed, the set is insensitive toany 1 signal not containing the sub-audio frequency.'i

selected by the selector buttons 94, .96 and x08.r

If the operator desires to operate-this set as an ordinary radio not selective to any particular program hemay open the switch |56 or the switch |58. If he opens Tthe switch |56 the signal received must be of sufdcient amplitude for satis factory listening or the grid |36' will not be biased beyond cutoff and a squelchl signal ywill remain on the grid |16 so as to make this V'tube inoperative. This also produces a noise squelching `action between stations. If the operator.

desires to receive all signals regardlessv of audio frequency amplitude he may open the switch |58. This switch completely de-energizes the squelch system andthe radio may be operated as Van ordinary manually tuned or .push button tuned set. Figure 2 of the drawing shows a modification of the present invention which includes means termined sub-audio modulation. Thisxmodiflcation may also be used to automatically tunev` the set to any station whose signal at the re-` ceiver is of suillcient amplitude to give satisfac-.w

tory reception. Y

Referring to Figure 2, 2 is a conventional antenna which 'is coupled to a radio receiver v225 by a conventional `coupling system. 225may be ka typical receiver capable of receiving eitherams the coupling circuit between them is of conven- Y tional design and of the same general Atype as more specifically described in connection withv Figure 1. Resistors 234 and 236 correspond respectively to resistors 36 and 31 in Figure 1. Figure l has a manual means for tuning the re-4 ceiver while in the present modification this.

same mechanical means is driven by an electric motor instead of manually. Motor drives are old in the art and therefore the .applicant has not considered it necessary to indicate specifically the exact connection between the tuning motor 238 and the mechanical linkage usedto drive the variable condenser or inductance for varying the resonant frequency of circuits in the radio frequency and local oscillator portion of.

The output across the resistors 234 the receiver. and 236 is, as more specifically described in Figure l, coupled to the amplier 6 by means of conductor 248. This amplifier 6 is identical to the amplifier 6 described in connection with Figurev l and therefore will not be more fully described at the present time. kThe sub-audio signal from the amplifier 6 is impressed at the point 242. 244, 246 and 248 contain band pass filters, each of which is responsive to a, preselected sub-audio code frequency. For example, the proportioningV of Icondensers 258, 252, 254 and inductances 256,

251 and 258 may be such as to pass a frequency of cycles per second. The proportioning of condensers 268, 262, 264 and inductances 266, 261 and 268 may be such as to pass a sub-audio frequency of 10 cycles per second. The proportionsof condensers 210, 212, 214 and inductances 216,

211 and 218 may be suchas to pass a sub-audio'.

frequency of cycles per second. With these assumed values, 244 will pass 5 cycles per second,

246 will pass 10 cycles per second and 248 will, pass 20 cycles per second. Selective buttons 288,.` 282 and 284 are included for the purpose of con-2 necting the desired sub-audio filter into the circuit to the tuning motor control 286. These selective buttons have a second purpose, in addition to connecting a lter into the circuit and,

that is, to close the switches 288, 298 or 292 vrespectively, momentarily on the depression of the-r The switches connecting the band pass lter into the circuitL remain closed selected button.

after the button is pushed. For example, when selective button 288 is depressed, switches 294 and 296 are closed permanently while contacts 288' are momentarily closed and then reopened. When selective button 282 is actuated, contacts 298 and 388 are permanently closed while ycontact 298 is momentarily closed and then reopened. When button 284 is depressed, contacts 382 and 384 are permanentlyv closed while contact 292 is only momentarily closed and thenreopened.

The depressing of button 288, 282 or 284 connects the selective band pass lter into the circuit and the sub-audio frequency thus passed is transferred by means of conductor 386 to the grid 388 of the triode section of the duo diode triode 318. 'Iibe 318 has cathode 312, a triode platel 314 and diode plates 316 and 318. The cathode 1 a ground return resistor for grid 388.` The subaudio frequency placed upon the grid 388 is amplied Aby thetriode `and impressed across load resistance 324. This signal is coupled to the diode plates 316 and 318 by means of coupling condenser 326 and resistor 328. The output of this duo diode is a combined negative unidirectional sub-audio and D. C. potential. The A. C. components of this signal are removed by low passlter including resistor 338 and condenser 332.

The remaining D. C. potential is impressed uponl the grid 334 of the double triode 336. This double,

triode 336 has cathodes 338 and 348 and plates 342 and 344 as well as a second control grid 344.

The cathodes 338 and 348 are maintained at the.

desired bias potential by means of adjustable biasing resistance 348 plus potential dropping resistor 358 and adjustable biasing resistor 352 plus potential dropping resistor 354 respectively. Since these cathodes 338 and 348 receive their bias from a source having no A. C. components it is not necessary to by-pass 348 or 352 with a condenser. Due to the circuit arrangement o1.' thediode rectier any signal impressed upon the grid 334 through conductor 356 is negative. By properly proportioning the biasing resistor 352 and potential dropping resistor 354 the potentials within this tube may be so adjusted that current may be allowed to pass from plate 344 to cath.

ode 348 when no D. C. potential is impressed upon the grid 334 but the tube will not conduct when a negative potential is impressed upon this grid. Referring now to the receiver 225, a second conductor 358 is adjustably connected to the same position on the resistor 234 as is conductor 248. The combinedaudio, sub-audio and D. C. po.- tentialproduced across resistors 234 and 236 by means of the detector action of the diode porA tion of duo-diode-triode 228 is impressed upon the conductor 358. All alternating frequency components present in the signal in conductorI and the cathode 338 when no negative potential is impressed upon the grid 346. The impressing of a negative potential on the grid 346 will,

however, bias this tube to beyond cutoi and` therefore prevent the flow of current from its plate to its cathode. The triode 364 may be termed a relay control triode since it controlsy the flow of current through the relay coil 333 and hence opens and closes the contacts 368 of the relay. Upon inspection of the circuit it may readily be seen current from the B plus power supply 318 may dow through resistors 312 and 314 to give the cathode 311 of the triode 364 the proper bias. The plate potential for this tube is obtained through the relay coil 366. The plate 316 of this tube, of course, draws no current when the grid 318 is negatively biased beyond cutoff. The negative bias for the grid 318 is obtained across resistor 380. If current is allowed to flow through the conductor 382 because either section of the double triode 336 is conducting, a voltage drop across the resistor 388 will cause the grid 3180i the tube V364 to be biased beyond cutoff. In this condition no current will ow through the relay coil 366 and the contacts 388 of the relay will be in the closed position thus' energizingy tlie tuning motor 238 through lcon The resistors 343 and 358 are so adjusted and proportioned that current will be conducted by the plate 342 .to the same potential as thev cathode 311.

ductors 384'and 386 from power source 388. This power source may be of any desirable-sourceca- 'pable of actuating the tuning motor. This tunthus prevent that half of the tube from 'ccnducting. If it is desired to use the presentinvention to receive any and all radio stations, a switch 380 is provided to permit the vcontrol of the system by the energization of only the grid 346. If, however, the system is used to selectively receive a desired chain radio program the switch 3381s closed so that the biasing ofthe vgrid 346 does not `preventthe flow of Acurrent through the conductorv 382.

When the tuning means sweeps across the radio frequency so that y'a signal is received havinga preselected subaudio code modulated thereon a current is caused to iiow through the conductor 306"and by means of the circuit previously described. This impresses a negative potential on the grid 334.'

Under these conditions both halves of the'double triode are biased beyond cutoff so that no current is allowed to flow ythrough the conductor 4382. Under these conditions no current flows through the resistor 386 and hence the grid v318 returns The removal of this negative bias permits the tube 364 to conduct thus permitting a current'to iiow from the power source 310 through the relay coil 366 through the plate 316y through the cathode 311 through the resistor 314 to ground. This energizes vthe relay, thus opening the contacts 368 and stopping the tuning motor 238 on" the frequency of the carrier containing the 4selected sub-audio modulation; If it is desiredl to' receive a station carrying a different chain program or to a diiferent station carrying the same chain program, the program selectorvbutton either 280, 282 or 284, as desired,`is depressed. This momentarily closes either contacts 288,T 29B or 292 respectively; thus shortin'g the relay coil 366 deenergizingV the relay and thus allowing the contacts 368 to close which reenergizes the tuning motor 238'to retunethe set;

l Operation of Figure Z If the operator of the radio receiver desires to obtain ay program being broadcast. by, sayg' the National Broadcasting Company and' this-'program has a sub-audio modulation of say','5 cycles per second', the operator presses the button 280. This closes the contacts 288 deenergizing therelay coil 366 allowing the contacts 368 to close thus starting the tuning motor 238. Immediately thereafter the contacts 28,8 reopen giving control of; the tuning motor to the tube 364. The' tuning motor continues to scan` the spectrum until a carrier having a modulation frequency of 5 cycles' per' second is reached. The'lter system 244 by passing this cycle signal causes the current to flow in the conductor 306 and impress a signal on. the grid of the tube 318. This signal isamplified'and causes a negative'bias to be impressed' upcn'the tube 334. Simultaneously this 5 cycle modulation frequency plus othermodulation frequencies on the received signal causes the negative grid bias to be impressed on the grid 34E. The `presence of this bias on both grids 346and 3341'causes` the stopping of current through the resi'stor'380 thus removing the negative'bias from the grid 318. This removal of grid bias allows the tube 364 to conduct thus energizing the coil 366 to open the relay contacts and stop the motor 238 on the received carrier frequency.

If the operator desires tov receive any radio signal having sufficient intensity to give satisfactory reception, he opens the contactiiil thus disabling the sub-audio frequency control part of the circuit. The circuit is then entirely responsive to theconduction or non-conduction of current through the plate 342. The operator the'n presses any one of the station selector buttons 280, '282 or 284 to momentarily shunt the relay coil 366 thus allowing the contacts 368 toI close energizing the tuning motor 238. As previously described; the contact 288, 230 or 232 immediately reopens giving control ofthe tuning means to the conductor 353. The tuning motor causes the tuning means to scan the frequency spectrum until a signal is received. The D. C. potential produced by rectification is then impressed on the 'conductor 358 and produces a negative grid bias on the grid 346. This bias is produced regardless of whether the received signal contains any subaudio code frequency or not. The presence of this negative grid vbias stops the conduction of the tube and hence the current through the con"- ductor 382 and the resistor 388. This removes the grid bias fromlthe tube v35e thus allowing this tube to conduct, producing a current through the relay coil 365. This opens the 'contacts 368 thus deenergizing the tuning motor and stopping the tuning means on the frequency producing the grid bias on the grid 346. If the operator desires to-` tune the set to a second station he merely pushes either one of the selectorbuttons andthe operation is repeated. f

It is well known that at very low frequencies vibrating reeds may beutilized to produce a band pass filter which may be cheaper and more eii'-- cient than the filters `using inductances and condensers, as for example those describedin ccn-A nection with Figures l and 2.

Therefore, referring to Figure 3, the amplifier 460 is'shown. This amplier receives the audio and sub-audio frequency components from the receiver through conductor 482. The conductor A402 corresponds to conductor 46 in lli'igure l and 248 in Figure 2. A low pass lter having resistor 404 and condenser M13 remove all audio frequency components; The coupling condenser 488 and 'ref sistor 4m remove the D. C'. components'and im-v press the sub-audio frequencies on the grid 4K2 of the beam power tube 414. This tube has a cathode 416, a second'grid 4m and a plate 428.' The self-bias for the cathode M6 is produced 'by the biasing circuit including resistor 42l and condenser 422. The sub-audio frequency output of thistube is impressed across the coil 424 and condenser 426 ofr the selector system 438. This mechanical selector system has reeds 433, 432` and 434 each having a mechanical resonant pewv riod ata desired sub-audio frequency. lThe reed 438, for example, may have a resonant periodlc'f 5 cycles per second, the reed 432 have'a resonant period of 10 cycles per second and the reed 434V havea resonant period of 20 cycles per second.` These reeds are magnetically coupled to the cfoil 424 and are capable of vibrating at their own resonant frequency when energized `by a signal of. that frequency. Selector buttons 436, 43il`and: 440are connectedl respectively to the vibrating reeds 438, 432 and-434. Whenone of thesefsea,

lector buttons is depressed it closes contact 4.42,'v

444er 446; This"completes the circuit throughy contact 448, 45B or 452. If the operator,for example, desires to receive a station having a subaudio modulation frequency of 5 cycles-per second he will depress the button 436 :thus closing contact 442. The tuningA means then scans through the spectrum until a signal having a 5 cycle sub-audio modulation is received. When this 5 cycle sub-audio signal is impressed on the coil 424 the mechanical resonance period of -the contact 43B causes this reed to vibrate in synchronism with the electrical signal in the coil 424. This periodically completes the circuit from the power source 454 through the contacts 442 through contact 448 to ground. This 5 cycle signal is carried through conductor 456 to the grid 102 in the squelch removing circuit as shown in Figure 1 or to the grid 388 in the tuning motor control circuit as shown in Figure 2. The condenser 458 is of such capacity as tol effectively bypass high frequency components caused by the vibrating contact 448 but not the sub-audio control signal frequencies of 20 cycles or less. If the operator desires to receive signals having a subaudio modulation of 10 cycles per second he depresses button 438. If he desires to receive sig- -nals having a sub-audio modulation of 20 cycles per second he depresses button 440.

It may thus be seen that the filter system shown in Figure 3 may be substituted for the filter system utilized in the circuits shown in Figure 1 and Figure 2.

It is to be understood that although the invention has been described with specific reference to a. particular embodiment thereof, it is not to be so limited. The present system, for example, could be adapted to operate on a coded predetermined number of pulses of sub-audio frequency and thus tune the radio to the desired frequency without departing from the broad scope'of the present invention. In such a system the station carrying the chain program would emit a predetermined number of sub-audio pulses modulated on the carrier. These pulses would actuate counters instead of filters in the receiver and the output of these counters would supply the stopping or squelch removing voltage,

It is therefore intended that the scope of the appended claims be construed as broadly as their language and the status of the prior art permits.

I claim:

1. A radio receiver capable of receiving a radio frequency carrier signal modulated by a subaudio frequency voltage and an audio frequency voltage; including a tuned amplifier capable of amplifying the carrier frequency, together with side bands, a detector capable of detecting both the audio and sub-audio frequency voltages from the output signal of said amplifier, a filter for separating said audio and sub-audio frequency voltages, means for adjusting the tuning element of said tuned amplifier and means responsive to both the carrier frequency signal and said subaudio frequency voltage for causing said adjusting means to be ineffective.

2. A radio receiver as claimed in claim 1, in which the filter for separating the sub-audible frequency voltage consists of mechanically vibrating reeds carrying electrical contacts,

3. A radio receiver as claimed in claim 1 in which the means for adjusting the tuninglelemen is an electrical motor.

4. A radio receiver as claimed in claim 1 in which the means responsive to both the carrier frequency signal and the sub-audio frequency voltage includes a thermionic valve which is ren;

iff

212 dered conductive by presence of both the carrier and the sub-audio frequency signals.

-5. In a radio receiver capable'ofreceiving a radio frequency carrier signal modulated with a fixed sub-audio frequency voltage and an audio frequency voltage, means for detecting said audio and sub-audio frequency voltages, a vacuum tube amplifier for amplifying said audio frequency voltage,` a grid in at least one tube of said amplifier normally negatively biased to make said amplifier inoperative, means vresponsive to the combined effect of said carrier frequency signal and said sub-audio frequencyfvoltage to remove said bias and permit said audio frequency amplifier to amplify said audio frequency voltage. I

6. A radio receiver capable of receiving a radio frequency carrier signal modulated with anA audio and a sub-audio frequency voltage including,` an electric motor for tuning said radio through'a range of said radio frequencies, a detector capable of detecting the sub-audio frequency voltage from said radio frequencycarrier, an electronic amplifier capable of amplifying said sub-audio frequency, a thermionic tube having a relay coil'in -the plate thereof,- relay contacts responsive to the current in said cOil for Vdeenergizing'said tuning motor and means responsive to said detected subaudio frequency forremoving control grid bias from said thermioinictube to allow said tube to conduct electricity and thus operate said relay.

7. A radio receiver as claimed in claim 6 in which the means responsive to the sub-audio frequency voltage includes a-'rectifier and a therm'- ionic tube the control grid of which is biased beyond cutoff by the' presence of said'rectifled sub-audio frequency voltage and which allows plate current conduction when said voltage is not present on said control grid.

8. A radio receiver'capable of receiving a radio frequency carrier signal modulated by both audio and sub-audio frequency voltages including; an electric motor for tuning the radio through a band of radio frequencies, a detector for detecting said audio and sub-audio' frequency voltages from said radio carrier frequency, means for rectifying said radio frequency carrier signal, an amplifier capable of amplifying said sub-audio frequency voltages, a plurality of filters'each capable of passing a predetermined sub-audio frequency, means for introducing a selected filter into the plate circuit of said sub-audio frequency amplifier, means for rectifying the selected sub-audio frequency', a thermionic tube, means responsive to the'plate current thereof for deenergizing said tuning motor, means responsive to the combined effect of the rectified audio and the selected sub-audio signals and for removing the grid bias of said tube to allow conduction of said plate current to deenergize said tuning motor.

9. A radio receiver as claimed in claim 8 in which the means responsive to the rectified signal consists ofvtwo thermionic tubes having a common plate circuit, one of said thermionic tubes being biased beyond cutoff responsive to the rectified sub-audio frequency and one of said tubes biased beyond cutoff bythe rectified carrier frequency signal in such a manner that current ceases to flow in the common plate circuit only when a received radio frequency signal contains sub-audio components so as to bias both said control grids beyond cutoff. 1

10, A radio receiver capable lof receiving a. radio frequency carrier modulated with an audio frequency including; an electric motor fortun ing-said radio through a range of radio frequencies, a rectifier capable of rectifying said radio frequency carrier signal, a thermionic tube having a relay coil in the plate circuit thereof, relay contacts responsive to the current in said coil to deenergize said tuning motor, and a second thermionic tube the control grid of which is biased beyond cutoff by the presence of said rectified signal and which allows plate current conduction when no signal is present on said control grid, the removal of said last mentioned plate current by said signal causing the removal of the control grid bias on said first mentioned thermionic tube to thereby allow said tube to conduct electricity and thus operate said relay.

11. A radio receiver as claimed in claim 1 in which the filter for separating the sub-audio frequency voltage includes mechanically vibrating reeds carrying electrical contacts and in which the means for adjusting the tuning element is an electrical motor.

12. A radio receiver as claimed in claim 1 in which the means responsive toboth the carrier frequency signal and the sub-audio frequency voltage includes a thermionic valve which is rendered conductive by the presence of both the carrier frequency signal and the sub-audio frequency voltage in which the means for adjusting the tuning element is an electrical motor.

13. A radio receiver as claimed in claim. 1 in which the filter for separating the sub-audio frequency voltage includes mechanically vibrating reeds carrying electrical contacts and in which the means responsive to both the carrier frequency signal and the sub-audio frequency voltage includes a thermionic valve which is rendered conductive by the presence of both vthe carrier frequency signal and the sub-audio frequency voltage.

14. A radio receiver as claimed in claim 1 in which the filter for separating the sub-audio frequency voltage includes mechanically vibrating reeds carrying electrical contacts and the means for adjusting the tuning element is an electrical motor, and the means responsive to both the carrier frequency signal and the subaudio frequency voltage includes a thermionic valve which is rendered conductive by the presence of both the carrier frequency signal and the sub-audio frequency Voltage.

15. A radio receiver capable of receiving a radio frequency carrier signal modulated with an audio and a sub-audio frequency voltage including, an electric motor for 'tuning said radio through a range of said radio frequencies, a detector capable of detecting the sub-audio frequency voltage from said radio frequency carrier, an electronic amplifier capable of amplifying said sub-audio frequency, a thermionic tube having a relay coll in the plate thereof, relay contacts responsive to the current in said coil for deenergizing said tuning motor and means responsive to both said detected sub-audio frequency and said radio frequency for removing control grid bias from said thermionic tube to allow said tube to conduct electricity and thus operate said relay.

BERTRAM A. SCHWARZ.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date Re. 21,151 Adair July 18, 1939 2,105,789 Linsell Jan. 18, 1938 2,144,822 Van Loon Jan. 24, 1939 2,197,933 Kirkwood et al Apr. 23, 1940 2,262,218 Andrews Nov. 11, 1941 2,410,075 Hutchins et al Oct. 29, 1946 

